This `web.HttpFileHandler(srv, "/provider.json", opts.ApiPath+"provider.json")`
should have been `opts.ApiPath+"/provider.json"`. The difference is there was a
missing / before provider.json. That results in vpnweb trying to open a path
that doesn't exist: /etc/leap/config/vpnprovider.json.

Because this is so hard to see, its better to use filepath.Join, so these things
cannot happen. So I've switched these to the standard library function that
joins paths.